Additive manufacturing of an Fe-Cr-Ni-Al maraging stainless steel: Microstructure evolution, heat treatment, and strengthening mechanisms

Additive manufacturing of a low carbon Fe-Cr-Ni-Al maraging stainless steel (with the brand name CX) through the laser-powder bed fusion (LPBF) process is studied. Since the strength of this material is enhanced through precipitation hardening, the effect of different heat treatment cycles on the hardness and microstructure is assessed. The LPBF-CX is heat treated through a standard heat treatment procedure consisted of austenitization at 900 degrees C for 1 h followed by air cooling and aging at 530 degrees C for 3 h. Moreover, the effect of aging treatment (with no austenitization) on the as-built sample is studied. The microstructure of the as-built, austenitized-aged, and aged samples is studied using multiscale electron microscopy techniques. The as-built LPBF-CX consists of the typical lath martensitic structure and minor retained austenite. The martensite laths are featured by high dislocation density, with no evidence of precipitates. Austenitization-aging treatment shows a detrimental effect on the strength of LPBF-CX, due to martensite laths growth and retardation of precipitates evolution. Aging of the as-built LPBF-CX results in strength enhancement due to the evolution of nanometric and coherent beta-NiAl precipitates, and martensite laths refinement. Moreover, the pre-existing dislocation networks play a key role in the strength of the aged material. The strength enhancement of the aged LPBF-CX is investigated through the fundamentals of alloy hardening.